Determination Of The Valency Of Magnesium 3ut35

Unit Title: Chemistry Laboratory I Unit Code: UDEC 1134 Name: Ooi Jiaxin ID: 1500257 Name of Lab Partners: Tan Jia Ying, Rita Sharmila Dewi Practical Group: 5 Experiment No.: 2 Experiment Title: Determination of the Valency of Magnesium Experiment Date: 03/02/2015 Submission Date: 24/02/2015 Lecturer’s Name: Ms. Chang Chew Cheen

Title: Determination of the Valency of Magnesium Objective: To study the quantitative relationship between the amount of reactants and products of a reaction. A known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry and the valency of magnesium. Introduction: Stoichiometry is the study of the quantitative aspects of chemical formulas and reactions (Silberberg, 2006). It is important in various study fields which demands the determination of the amount of substance(s) that need to be used in a reaction or production of new substances. In this experiment, the valency of magnesium is to be determined. This was done by the reaction of magnesium ribbon with hydrochloric acid and thus the amount of hydrogen gas liberated from this reaction can be determined. The reaction equation is as follow:

Mg (s) + XHCl (aq) →MgClx (aq) +

X 2 H2 (g)

For this experiment, the amount of magnesium ribbon that undergoes the reaction is made known but the hydrochloric acid was provided in excess because magnesium will react with the protons in HCl to form magnesium ions and hydrogen gas. Therefore, the amount of hydrogen gas and magnesium chloride produced depends on the amount of magnesium used in this experiment. A comparison of the amount of hydrogen gas produced with the amount of magnesium consumed will enable the X value to be determined. Apparatus/Glasswares: Burette, pipette, retort stand, electrical balance, watch glass, beaker, gauze, funnel, glass rod, thermometer. Chemicals/Reagents: Magnesium ribbon, 0.5 M HCl Procedure: The burette is going to be used upside down to collect the hydrogen gas. But first, the unmarked space between the 50 cm³ mark and the tap of unknown volume is to be determined. Hence, 25.00 cm³ of distilled water is pipetted into a clean and dry 50 cm³ burette which was clamped vertically (right way up). The burette reading was noted, and the burette was drained and the test was repeated. The water was left in the burette for 10 minutes and leaks in the burette was checked. A piece of magnesium ribbon was cleaned with steel wool. A piece of the magnesium ribbon was cut with scissors within the lengths shown. The ribbon was curled up. A watch glass was put on a four decimal balance and the magnesium ribbon on the watch glass was

weighed accurately between 0.0300g and 0.0360g. The watch glass with magnesium ribbon on top was then put inside a 600 cm³ beaker. A small filter funnel with short stem (1.0 – 1.5 cm long) was taken and covered with gauze. It was inverted and placed on the watch glass over the magnesium. The beaker was carefully filled with tap water until the level of the water is approximately 0.5 – 1.0 cm above the end of the funnel stem. The burette was filled completely with 0.5 M HCl, and it was inverted with supplied cork pressed flat over the open end. The inverted burette was placed in the water in the beaker. The cork was removed and the end of the burette was placed over the stem of the funnel while ensuring no air can enter the burette. The end of the burette was clamped into position. The excess water in the beaker was removed with a pipette until the level of water was just above the stem of the funnel. 100 cm³ of 0.5 M HCl was added to the beaker. A glass rod was used to stir and mix the mixture to ensure that HCl has reached the magnesium. The watch glass was also tapped using the glass rod. The solution was stirred to initiate the reaction and stirring has stopped so that the reaction proceeded without help. The magnesium escaped from the fine gauze and funnel into the burette but reaction with hydrochloric acid still occurred. The watch glass was tapped gently at the completion of the reaction between magnesium and hydrochloric acid to dislodge any gas bubbles. Results and Calculations: Unmarked volume of burette = 25.0 cm³ - (50.0 – 29.5) cm³ = 4.5 cm³ Mass of magnesium ribbon = 0.0148 g Final reading of burette after experiment = 37.8 cm³ Total volume of hydrogen gas produced = 4.5 cm³ + (50.0 – 37.8) cm³ = 16.7 cm³ Calculation of X:

Given that the equation: Mg (s) + XHCl (aq) → MgClx (aq) +

The number of moles of magnesium = =

X 2 H2 (g)

mass of magnesium molar mass of magnesium 0.0148 g 24 g mol−1

= 6.167 x 10-4 mol

Actual mole of HCl used = =

MV 1000 0.5 x 100 1000

= 0.05 mol

Number of moles of HCl : Number of moles of Mg = 0.05 mol : 6.167 x 10-4 mol = 81.08 : 1 Theoretical number of moles of H2 gas 0.0167 dm 3 = 24 dm 3 mol−1 = 6.958 x 10-4 moles Number of moles of Mg : Number of moles of H2 = 6.167 x 10-4 mol : 6.958 x 10-4 mol = 1 : 1.128 ≈ 1 : 1

∴

X 2

=1

X=2 Substituting X = 2 into reaction equation: Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) Questions and Discussion: 1. It is advisable that the temperature is not taken for at least 20 minutes after adding the HCl. Why is this so? The moment the HCl was added, the dissociation of HCl to form H + ions and Cl- ions results in the absorption of heat from the solution in beaker (endothermic reaction). The magnesium then formed magnesium ion and then magnesium chloride whereas the H+ ions form hydrogen gas and as that happened, heat is released to the solution in the beaker (exothermic reaction). Hence, the final temperature of the solution can be determined after 20 minutes. 2. Calculate the moles of hydrogen gas present using the given calculation method.

Number of moles of hydrogen gas =

0.0167 dm 3 24 dm 3 mol−1

= 6.958 x 10-4 mol 3. Give the ideal gas equation and specify what each variable is. Show one mole of gas at S.T.P. occupies 22.4 L. Ideal gas equation: PV = nRT; where P = pressure, V = volume, n = number of moles, R = gas constant (8.3145 J mol-1 K-1 or 8.206 x 10-2 L atm K-1 mol-1), T = temperature Given 1 mole of gas at standard temperature and pressure, S.T.P., n = 1.0 mol At S.T.P., P = 1.00 atm and T = 0 C = 273.15 K From ideal gas equation, PV = nRT nRT V= P Substitute values of P, n, R and T into equation, 1 .0 mol x 8.206 x 10−2 L atm K −1 mol−1 x 273.15 K V= 1.00 atm = 22.4 L (shown) 4. What will be the result if hydrogen gas, H2, leaks through the stopcock of the inverted burette? The calculated volume of hydrogen gas liberated in the reaction will be lower. The number of moles of magnesium to the number of moles of hydrogen gas ratio will not be 1:1. The actual amount of magnesium ribbon will be assumed to produce less amount of hydrogen gas. In this experiment, magnesium ribbon reacted with hydrochloric acid to form hydrogen gas. Magnesium, being an alkaline-earth metal, reacts with any acid to form magnesium salts and liberate hydrogen gas. Hydrogen chloride is an acid gas, but exists as hydrochloric acid in aqueous solution. When it reacts with magnesium, it dissociates to form hydrogen ions and chloride ions. Magnesium dissociates to form magnesium ions and release electrons during the process. HCl (aq) → H+ (aq) + Cl- (aq) Mg (s) → Mg2+ (aq) + 2e- (aq)

In ionic reactions, metals release or donate electrons to form positive ions or cations and the electrons are accepted by the non-metals involved in the reaction to form negative ions or anions. In this experiment, magnesium being the metal donates 2 of its electrons to two chloride ions to form Mg2+ and 2 Cl- ions. Mg2+ (aq) + Cl- (aq) → MgCl2 (aq) Magnesium chloride, MgCl2, exists as a colourless solution in the solution in the beaker. The hydrogen ions formed reacted with the electrons released by magnesium to form hydrogen gas, H2, which was liberated and formed inside the burette. 2H+ (aq) + 2e- (aq) → H2 (g) Precaution Steps: When handling the 0.5 M hydrochloric acid, gloves were worn as HCl is an irritant and cause the skin to itch. The proper techniques of handling the burette when putting it into the water were also carried out to prevent the entry of air and air bubbles into the HCl-filled burette. This action was necessary as to prevent any errors in calculating the production of hydrogen gas collected which was then compared with the amount of magnesium used (number of moles of magnesium to number of moles of hydrogen gas ratio). Moreover, the readings on the burette and other necessary measuring equipment such as the measuring cylinder in measuring 100 cm³ of hydrochloric acid were taken by reading with the eye level on the same level as the level of the solution or chemical(s) in order to prevent parallax error. Conclusion: The value of X is 2 and the reaction equation can be written as follows: Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) The volume of hydrogen gas produced is 16.7 cm³ through the reaction of hydrochloric acid with 0.0148 g of magnesium ribbon. Magnesium chloride and hydrogen gas are produced in the reaction and no ions are left in the solution as all of the ions reacted to form respective compounds or gas. Reference: 1. One Part of Chemistry, 2011. Determination of the Valency of Magnesium, [Blog] 7 June 2011. Available at: http://1chemistry.blogspot.com/2011/06/determination-of-valency-ifmagnesium.html?m=1 [Accessed 22 February 2015].

2. Silberberg, 2006. Chemistry The Molecular Nature of Matter and Change. 4th ed. New York:

McGraw Hill.